linux-kernel - Re: [PATCH v2] slab: Add POISON_POINTER_DELTA to ZERO_SIZE

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Message-ID: <CAGXu5jKC8XTP=gjCGQYEEwSQEAWM66E8HedaEqZR3F=QSm+aTg@mail.gmail.com>
Date:   Fri, 18 Nov 2016 09:55:08 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Christoph Lameter <cl@...ux.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR

On Fri, Nov 18, 2016 at 9:47 AM, Christoph Lameter <cl@...ux.com> wrote:
> On Thu, 17 Nov 2016, Michael Ellerman wrote:
>
>> Currently ZERO_OR_NULL_PTR() uses a trick of doing a single check that
>> x <= ZERO_SIZE_PTR, and ignoring the fact that it also matches 1-15.
>
> Well yes that was done so we do not add too many branches all over the
> kernel.....

There are actually very few callers of this macro. (Though it's
possible they're executed frequently.)

>> That no longer really works once we add the poison delta, so split it
>> into two checks. Assign x to a temporary to avoid evaluating it
>> twice (suggested by Kees Cook).
>
> And now you are doing just that.

In this case, what about the original < ZERO_SIZE_PTR check Michael
suggested? At least the one use in usercopy.c needs to be fixed, but
otherwise, it should be fine?

-Kees

-- 
Kees Cook
Nexus Security